This invention relates to a fluidized bed heat exchanger and, more particularly, to a heat exchanger in which heat is generated by the combustion of particulate fuel in a fluidized bed.
The use of fluidized beds has long been recognized as an attractive way of generating heat. In a normal fluidized bed arrangement, air is passed through a perforated plate, or grid, which supports a bed of particulate material, usually including a mixture of fuel material, such as high sulfur bituminous coal, and an adsorbent material for the sulfur released as a result of the combustion of the coal. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. In addition to enjoying a capability for considerably reducing the amount of sulfur-containing gases introduced to the atmosphere, such an arrangement permits relatively high heat transfer rates, substantially uniform bed temperatures, combustion at relatively low temperatures, a reduction in corrosion and boiler fouling and a reduction in boiler size.
In the fluidized bed combustion process, the coal and adsorbent are continuously introduced into the bed by suitable feeders, injectors, or the like and the spent coal and adsorbent are discharged from the lower portion of the bed, usually through a gravity drain pipe extending through a wall of the heat exchanger or through a discharge opening formed through the perforated support plate. However, in these type of arrangements, there is only a minimal number of feeders or injectors, which are usually mounted through the heat exchanger walls, which results in poor distribution and mixing of the particulate material in the bed and therefore an insufficient residence time and a reduced reaction efficiency. This problem is specially acute in connection with particulate coal of high reactivity, such as lignite, and/or relatively fine size particles (smaller than 1/8 inch), and/or low density particles, all of which require good mixing and distribution for maximum efficiency. Conversely, the feed of larger sized coal may leave larger sized ash particles which can be difficult to drain from the bed.